Simon Montefiore was born in London. His father was Stephen Eric Sebag Montefiore and his brother is Hugh Sebag-Montefiore. They are descended from a line of wealthy Sephardi Jews who were diplomats and bankers all over Europe and who originated from Morocco and Italy. After the Edict of Expulsion was issued against the Jews in Spain of 1492, some of Montefiore's ancestors stayed in the country whilst remaining secretly Jewish. During the reign of Philip II of Spain, one of them became governor of a province of Mexico, where he and his family were denounced by a political rival and tortured by the Inquisition. Two teenaged girls were burned alive in Mexico City while a son escaped to Italy and changed his name to Montefiore.
At the start of the 19th century, Montefiore's great-great-uncle, Sir Moses Montefiore, was an international financier who worked with the Rothschild family and who became a philanthropist. His mother, Phyllis April Jaffé, comes from a Lithuanian Jewish family of scholars. Her parents fled the Russian Empire at the beginning of the 20th century. They bought tickets for New York City, but were cheated, being instead dropped off at Cork, Ireland. Due to the Limerick boycott in 1904 his grandfather Henry Jaffé left the country and moved to Newcastle, England.
Montefiore was educated at Ludgrove School and Harrow School where he was an editor of the school newspaper, The Harrovian; he interviewed Margaret Thatcher for The Harrovian in the autumn of 1983.. He read history at Gonville and Caius College, Cambridge where he received his Doctorate of Philosophy (PhD). He won an Exhibition to Caius College.
Montefiore worked as a banker, a foreign affairs journalist, and a war correspondent covering the conflicts during the fall of the Soviet Union.
Montefiore's book Catherine the Great & Potemkin was shortlisted for the Samuel Johnson Prize, the Duff Cooper Prize, and the Marsh Biography Award. Stalin: The Court of the Red Tsar won History Book of the Year at the 2004 British Book Awards. Young Stalin won the LA Times Book Prize for Best Biography, the Costa Book Award, the Bruno Kreisky Award for Political Literature, Le Grand Prix de la Biographie Politique and was shortlisted for the James Tait Black Memorial Prize. Jerusalem: The Biography was a number one non-fiction Sunday Times bestseller and a global bestseller and won The Book of the Year Prize from the Jewish Book Council. His latest history is The Romanovs, 1613–1918.